To obtain high molecular weight polycondensation polymers, such as polyesters and polyamides, the polymerization process is conducted in two stages. In the first stage, a melt phase polymerization process is used to achieve an intermediate molecular weight, followed by the second stage which is a solid state polymerization (SSP) process which produces the final high molecular weight polymer. A melt process can be used to achieve the high molecular weight, but as the melt viscosity of the melt increases rapidly towards the end of the melt process, undesired side reactions can occur which limit the quality of the polymer. The production of a high molecular weight polymer in the SSP process can substantially avoid these side reactions and thus is the preferred process for high molecular weight products for critical applications such as bottle resins, industrial yarns and the like.
After the completion of the first process stage the melt polymerized polymer is extruded into strands, which are then cut into a pellet like form and are quenched, dried and transferred to a continuous solid state polymerization process. This comprises one of more crystallizers followed by a solid state reactor. The crystallizers heat and crystallize the polymer to a sufficiently high level of crystallinity such that it does not significantly crystallize further in the solid state reactor. Crystallization is an exothermic process that could lead to local hot spots in the solid state reactor if a polymer with too low a degree of crystallinity were fed to the reactor
The solid state reactor comprises a long vertical, or horizontal or nearly horizontal, vessel, which may contain an agitator, and/or which may rotate, in which the hot crystallized polymer pellets enter from the top of the vertical reactor or the inlet side of the horizontal reactor. Within the reactor body the pellets meet a counter flow of a hot inert gas, such as nitrogen. Inert gas means a gas that does not chemically react with the polymer during the SSP process, nor react with the by-products from the SSP process to form compounds that will chemically react with the polymer or affect the cleaning process of the inert gas. The temperature of the gas in the solid state reactor is typically 10 to 50° C. below the melting point of the polymer. This gas stream removes the polycondensation by-products such that the polymerization process is driven to higher molecular weights. After exiting the solid state reactor the polymer pellets are cooled, either by a cold gas or in a water slurry/spray system or other suitable heat transfer systems, dried, and finally transported to a storage silo. Preferably the inert gas discharge is passed to a scrubber or absorber unit where reaction by-products are removed.
The rate of the polymerization process in SSP is controlled by the ratio of polymer to gas flow, the gas temperature and the temperature of the polymer in the reactor and the desired molecular weight is achieved after the specified reaction time considering all these process variables. Control of this process is normally achieved by taking samples of the solid state pellets and measuring their solution or melt viscosity in a laboratory, which typically takes between 4 to 10 hours. If changes to the process variables are required to bring the molecular weight back to target, this involves either changing the ratio of polymer to gas flow, changing the temperature of the inert gas, changing the composition of the gas, changing the inventory of pellets in the reactor, or a combination of any of these parameters.